Large crankshafts, such as those utilized in marine main propulsion engines can exceed 20 meters in overall axial length and weigh in excess of 300 tonnes. A large crankshaft comprises a series of crankpins (pins) and main journals (mains) interconnected by crank webs (webs) and counterweights. The diameter of the journals can be as long as 75 mm (3 inches) and can exceed 305 mm (12 inches). Large crankshafts are heated and hot formed, for example by a hot rolling or forging process, which is favored over rolling. Steel forgings, nodular iron castings and micro-alloy forgings are among the materials most frequently used for large crankshafts. Exceptionally high strength, sufficient elasticity, good wear resistance, geometrical accuracy, low vibration characteristics, and low cost are important factors in the production of large crankshafts.
One known process for manufacturing large, or non-unitarily forged, crankshafts is diagrammatically illustrated, in part, in FIG. 1(a) through FIG. 1(g). The term “non-unitarily forged” is used since the massive size of large crankshafts, and other irregularly shaped large axial shaft components do not permit forging of the entire crankshaft at one time, as is done, for example, with smaller crankshafts used in the internal combustion engines of automobiles. The feedstock, workpiece or blank 10 used in the process is typically a drawn cylindrically shaped blank as shown in cross section in FIG. 1(a) at ambient temperature. Blank 10 may be, for example, a steel composition having an overall longitudinal (axial) length, L, of 20 meters and weight of 200 tonnes. Initially as shown in FIG. 1(b) a first pre-forge section 12a (shown crosshatched) of blank 10 is positioned within multiple turn induction coil 20 as diagrammatically illustrated in cross section. Alternating (AC) current is supplied to the induction coil from a suitable source (not shown in the drawings) to generate a magnetic field that couples with pre-forge section 12a to inductively heat pre-forge section 12a to a desired pre-forge temperature. Upon achieving the desired temperature in pre-forge section 12a, blank 10 is transported to a forging press (not shown in the figures) to forge an appropriate crankshaft feature or component, such as a first main journal or crankpin journal (referred to as the “first journal 12”). Forging temperatures typically used for steel compositions can range between 1093° C. to 1316° C. (2000° F. to 2400° F.). Subsequent to forging first journal 12, entire blank 10 is cooled down to near ambient temperature. Second pre-forge section 13a (shown crosshatched) of the blank is then positioned within the induction coil to heat pre-forge section 13a to forge temperature as shown in FIG. 1(c). Similar to the process for first pre-forge section 12a, second pre-forge section 13a is forged as second journal 13, after which the entire blank is again cooled down before heating the next section of the blank for forging. The process steps of section heating; section forging; and blank cool down are sequentially repeated for each subsequent feature of the large crankshaft, for example, as illustrated in FIG. 1(d) through FIG. 1(g) for journals 14 though 17.
Cool down of the entire blank after each section forging is driven by the necessity of having the same initial thermal conditions throughout the longitudinal length of the next section to be pre-forge heated so that the induction heating process heats the next section to a substantially uniform temperature throughout the longitudinal length of the next section. Without the cool down step, heat from the previous (last) forged section will axially flow by thermal conduction into the next section to create a non-uniform temperature distribution profile across the axial length of the next section, which will result in a non-uniform temperature distribution profile across the length of the next section after it is inductively heated within induction coil 20. These cool down steps are both time consuming and energy inefficient since heat energy dissipation to ambient in the cool down steps represents a non-recoverable heat and energy loss. Consequently overall energy consumption is dramatically increased with substantial reduction in overall process efficiency.
FIG. 2(a) through FIG. 2(d) illustrate the effects of an insufficient cool down of the blank after each section pre-forge heat step described in the FIG. 1(a) through FIG. 1(g) process. Depending upon the mass of the blank; material composition of the blank; and required pre-forge final temperature, it could take from around 30 minutes to more than 60 minutes to inductively heat the first pre-forge section 12a of the blank as shown in FIG. 2(a). Due to thermal conduction, there will be a substantial quantity of heat flowing from inductively heated high temperature pre-forge section 12a towards the end of the blank at a cooler (ambient) temperature. Upon completion of the first heating stage for pre-forge section 12a shown in FIG. 2(a), the blank is transported to the forging apparatus for forging the crankshaft feature in heated pre-forge section 12a. Typically the transport-to-forge apparatus step consumes several minutes. Additionally it also takes several minutes to forge the heated pre-forge section of the blank into the required crankshaft feature, and then several more minutes to transport the blank back to the induction coil for coil insertion and heating of the next pre-forge section 13a of the blank as shown in FIG. 2(b). Consequently during the forging and transport steps there is an appreciable time period for thermal conduction of heat from the already heated hot sections towards the cooler (unheated) sections of the blank, and when the next pre-forge section is positioned within induction coil 20, for example, pre-forge section 13a, as shown in FIG. 2(b), there will be a substantial residual heat concentration in pre-forge section 13a before induction heating thanks to axial heat conduction (illustrated by the “HEAT” arrows in the figures) from forged section 12 to pre-forge section 13a. More importantly the heat concentration in pre-forge section 13a will produce an appreciably non-linear initial temperature distribution along the length, L13, of pre-forge section 13a. 
Furthermore during the induction heating step of pre-forge section 13a, previously heated and forged first journal 12 (shown in dense crosshatch in FIG. 2(b) to indicate above ambient heated temperature) will serve as a source of heat with conduction heat flow towards next pre-forge section 13a, which will affect, in a non-linear manner, both transient and final temperature distributions in the blank, including the temperature uniformity of inductively heated pre-forge section 13a. Similarly upon completion of the heating and forging steps for second journal section 13, and prior to the heating step for next pre-forge section 14a as show in FIG. 2(c), there will be further, and more complex, heat flow gradients within the not-yet-forged sections of the blank due to thermal conduction. The initial temperature profile prior to induction heating of pre-forge section 14a of the blank is formed by complex thermal flow patterns in the blank resulting from the sequence of heating; transport-to-forge apparatus; forging; and transport-to-coil steps associated with forming first and second journals 12 and 13 as shown in FIG. 2(c). Non-uniformity of the initial temperature distribution prior to induction heating of the next pre-forge section 15a will further increase due to the cumulative impact of the previously heated and forged first 12, second 13 and third 14 journals of blank 10 as shown in FIG. 2(d).
FIG. 3(a) through FIG. 3(f) further illustrate the effect of the initial temperature on the final thermal conditions of blank 10 without cool down after each induction heating and forging steps for a section of the blank with the process described in FIG. 1(a) through FIG. 1(g). As shown in FIG. 3(a) at the beginning of the heating cycle, pre-forge section 12a is positioned inside of multiple turn induction coil 20. AC current is supplied to the induction coil from a suitable source (not shown in the drawings) to generate a magnetic field that couples with pre-forge section 12a to inductively heat pre-forge section 12a. Points, or nodes 112 through 312 (subscripts indicating sections in which the nodes are located), as illustrated in FIG. 3(a), represent typical critical nodes at the surface of pre-forge section 12a, which requires uniform heating by induction prior to forging. Node 413 is in section 13 of the blank located in proximity to the required uniformly heated pre-forge section 12a. Initial axial temperature distribution (TINITIAL12) prior to start of the induction heating step for first pre-forge section 12a is uniform, and typically corresponds to ambient temperature. The surface node locations versus temperature graph in FIG. 3(b) shows an initial temperature distribution (TINITIAL12) in the axial direction, and a required surface temperature distribution (TFINALREQ) at the end of the induction heating step for pre-forge section 12a. As described above, after the completion of induction heating of pre-forge section 12a, the sequence of transport-to-forge apparatus; forging; and transport-to-coil for the next section heating steps are performed, after which pre-forge section 13a will be positioned within induction coil 20 as shown in FIG. 3(c). During the time consumed by the above process steps, thermal conduction flow along the longitudinal axis results in a substantially non-uniform initial temperature distribution (TFINAL13) prior to the start of the induction heating step for second pre-forge section 13a as shown in the surface node locations versus temperature graph in FIG. 3(d). Temperature distribution (TINITIAL13) will be substantially non-uniform and appreciably different from temperature distribution (TINITIAL12). The initial temperature at node 113 (T1) in the FIG. 3(d) graph will be appreciably greater than the temperatures at nodes 213 (T2), 313 (T3) and 414 (T4); generally, T1>T2>T3>T4>(TINITIAL12). If the induction heating process for pre-forge section 13a is the same as that used for pre-forge section 12a, the final temperatures (TFINALACTUAL) at the representative nodes will be noticeably higher then the required temperatures (TFINALREQ) as graphically shown in the FIG. 3(d).
Process parameters playing a dominant role in the final temperature after the induction heating of each pre-forge section include: initial temperature of the pre-forge section; physical properties of the blank (primarily the specific heat value of the blank's composition); induced power in the pre-forge section; total induction heating time of the pre-forge section; and thermal surface losses from the blank due to heat convention and thermal radiation, which can be calculated from the following equation:
                              T          FINAL                =                              T            INITIAL                    +                      (                                                            P                  IND                                ×                                  T                  IND                                                            m                ×                c                                      )                    -                      Q            SURF                                              [                  equation          ⁢                                          ⁢                      (            1            )                          ]            
where TIND is the time (in seconds) of induced heating; PIND is the power (in kW) induced in the pre-forge section; m is the mass (in kg) of the inductively heated pre-forge section; c is the specific heat (in J/(kg·° C.)) of the blank's material composition, and QSURF is the surface heat losses (in ° C.) including radiation and convection. Equation (1) illustrates that there is a direct correlation between final temperature TFINAL and initial temperature TINITIAL, assuming all other factors remain the same.
When pre-forge section 13a absorbs a sufficient amount of induced heat energy during the heating step shown in FIG. 3(c), blank 10 is removed from induction coil 20 and is transported to the forging apparatus (not shown in the drawings) to forge second journal 13, after which the blank is transported back to the induction coil for heating of next pre-forge section 14a as shown in FIG. 3(e). However initial temperatures at nodes 114 through 314, and 415 will now be appreciably higher as illustrated in the surface node locations versus temperature graph in FIG. 3(f). With the process described in FIG. 1(a) through FIG. 1(g) this overheating will be further aggravated, and initial thermal conditions, (TINITIAL14), prior to induction heating of the next pre-forge section will cause further increase in the final temperature (TFINALACTUAL) compared to the required final temperature (TFINALREQ) as graphically shown in FIG. 3(f). Overheating can result in irregularities such as grain boundary liquation, metal loss due to excessive oxidation and scale, decarburization, improper metal flow during forging, forging defects (for example, crack development), or excessive wear of forge dies. Any of these irregularities can result in degraded performance of the forged article of manufacture.
Therefore with the conventional process described above, an uncertainty in the initial thermal profile along the longitudinal axis of the blank prior to heating the second, third, and successive pre-forge sections of the blank can lead to undesired thermal conditions in the pre-forge sections, including lack of temperature uniformity along the longitudinal axis in a pre-forge section. In the conventional process described above, this is avoided by the inefficient step of cool down after forging of each pre-forge section before induction heating of the next pre-forge step.
One object of the present invention is to produce a non-unitarily forged article of manufacture, such as a large crankshaft from a blank, or other large shaft article with a plurality of irregularly shaped cylindrical components, by sequential induction heating of each pre-forge section without the necessity of cooling down the crankshaft after forging each heated pre-forge section, by utilizing the heat absorbed in the blank during previous cumulative heating steps and reducing the required energy consumption.